public class Calendar
{
    [Key]
    [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
    public Guid ID { get; set; }


    public virtual IList<Appointment> FreeSlots { get; set; }
    public virtual IList<Appointment> AppointmentsList { get; set; }
    ...
}

public class Appointment
{
    [Key]
    [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
    public int AppointmentID { get; set; }
    public DateTime Start { get; set; }
    public DateTime End { get; set; }
    public String Type { get; set; }
    public String Info { get; set; }
    public Guid CalendarID { get; set; }
    public virtual Calendar Calendar { get; set; }
 }

modelBuilder.Entity<Appointment>().HasKey(u => new {u.AppointmentID, u.CalendarID });
        modelBuilder.Entity<Appointment>().HasRequired(u => u.Calendar).WithMany(c => c.FreeSlots).HasForeignKey(f => f.CalendarID).WillCascadeOnDelete(true);
        modelBuilder.Entity<Appointment>().HasRequired(u => u.Calendar).WithMany(c => c.AppointmentsList).HasForeignKey(f => f.CalendarID).WillCascadeOnDelete(true);

modelBuilder.Entity<Appointment>().HasKey(u => new {u.AppointmentID, u.CalendarID });
        modelBuilder.Entity<Calendar>().HasMany(c => c.FreeSlots).WithRequired(c => c.Calendar).HasForeignKey(c => c.CalendarID).WillCascadeOnDelete();
        modelBuilder.Entity<Calendar>().HasMany(c => c.AppointmentsList).WithRequired(c => c.Calendar).HasForeignKey(c => c.CalendarID).WillCascadeOnDelete();

modelBuilder.Entity<Calendar>()
    .HasMany(c => c.FreeSlots)
    .WithRequired(c => c.Calendar1)
    .HasForeignKey(c => c.Calendar1ID)
    .WillCascadeOnDelete(false);

modelBuilder.Entity<Calendar>()
    .HasMany(c => c.AppointmentsList)
    .WithRequired(c => c.Calendar2)
    .HasForeignKey(c => c.Calendar2ID)
    .WillCascadeOnDelete(true);